Light emitting diodes (LEDs) which emit light spontaneously under forward bias conditions have a variety of applications such as indicator lights, elements of visual displays, light sources for optical data links, optical fiber communication, and others. Of special interest for use in optical fiber communications are devices in which the light is emitted from the top surface of the device.
Prior art LEDs, used for optical fiber communication, typically emit light through an aperture in a top electrode which is upon a substrate layer of the device. A typical prior art LED includes in a descending sequence a top electrode, a substrate, a top confining layer, an active layer, a bottom confining layer, a bottom contact layer, a centrally located bottom electrode of relatively small area, a dielectric layer upon the remainder of the bottom contact layer, and a heat-sink. The top electrode has a centrally located aperture through which the spontaneous light emission takes place. The light emission, from the LED may be picked-up by an optical fiber, an end of which may abut the surface of the substrate within the electrode aperture. To catch greater proportion of the emitted light, the LED may be provided with a vertical well which is etched coaxially in the substrate down to the surface of the confining layer; this enables one to bring an end of the optical fiber closer to the source of the emission. In another version, a lens may be integrally formed in the surface of the substrate to capture and focus into the core of the optical fiber the light being emitted through the circular opening in the top electrode. For example see S. M. Sze, Semiconductor Devices, Physics and Technology, John Wiley & Sons, New York, 1985, pp. 258-267, and an article by Niloy K. Dutta, "III-V Device Technologies For Lightwave Applications", AT&T Technical Journal, Vol. 68 No. 1, January-February 1989, pages 5-18.
Unfortunately, the present-day LEDs suffer from numerous deficiencies. Light emission in the LED is spontaneous, and, thus, is limited in time on the order of 1 to 10 nanoseconds. Therefore, the modulation speed of the LED is also limited by the spontaneous lifetime of the LED. This limits the maximum modulation frequency to f.sub.max =200-400 Mbit/s. Next, light emission in the active region is isotropic, that is in all directions, such that only a fraction of the emission may leave the body through the opening in the top electrode. Spectral linewidth of the LED is broad, of the order of 1.8 kT where kT is the thermal energy. This results in chromatic dispersion in optical multimode fibers, i.e., pulse broadening, which limits the maximum distance of transmission of light emitted by an LED to a few kilometers at high transmission rates.
Attempts were made to improve the performance of the LEDs. For example an LED disclosed in U.S. Pat. No. 5,048,035 issued Sep. 10, 1991 to Hideto Sugawara et al. represents an attempt to increase emission of light from the top surface of the LED by providing a special current blocking semiconductor layer between a centrally located top electrode and the luminescent cavity so as to have a higher light extraction efficiency and luminescence. The current from the top electrode is widely spread by current blocking layer over the light emitting region leading to higher light extraction and higher luminance than with conventional LEDs. This LED includes in an ascending order a bottom electrode, a substrate, a bottom layer, an active layer, a top confining layer, a current blocking layer, a dot-like top contact layer, and a dot-like top electrode overlaying the contact layer. Except for the top electrode area, the light emission takes place from the upper semiconductor surface and not through the substrate. However, while such an emission is suitable for display and LED lamps, this emission is not suitable for optical fiber communication requiring a narrow line width of spontaneous emission.
Another attempt to improve luminescence output of an LED is described in an article by T. Kato et al., "GaAs/GaAlAs surface emitting IR LED with Bragg reflector grown by MOCVD", Journal of Crystal Growth, Vol. 107 (1991) pages 832-835, North Holland. The structure of this LED resembles generally the structure of the Sugawara et al. device. Namely, in the structure, the substrate is at the bottom of the device, and a top electrode is in the center of the top surface of the device. However, in this device the current blocking layer is absent, the contacting layer overlies all of the surface of the top confining layer, and the top surface of the contacting layer is coated with an antireflection layer to prevent reflections from the top surface-air interface. This LED also includes a multilayer distributed Bragg reflector (DBR) positioned between the substrate and the bottom confining layer of the device. The purpose of the DBR is to reduce absorption of light emission by the substrate. In this device, as in the Sugawara et al. device, the emission takes place from the periphery of the LED and, except for the area covered by the top electrode, primarily from the top surface. While such an emission is suitable for display and LED lamps, this emission is also not suitable for optical fiber communication.
Therefore, it is desirable to design an LED with improved light emitting characteristics suitable for optical fiber communications.